Nondestructive testing using air-coupled acoustic excitation

Abstract

An object is tested for defects by interferometry, by comparing images of the object taken under stressed and unstressed conditions. The stress is applied by perturbing the object with acoustic waves, produced by a speaker directed towards the object, without any mechanical coupling to the object. The acoustic energy can be of a single frequency, or it can be distributed over a set of random frequencies (i.e. white noise), or it can be in the form of a signal which is "swept" through a range of frequencies. In the latter case, the results can be stored in a video buffer which records the maximum signal obtained, for each pixel, while the signal is swept through the frequency range. Different defects in the object may resonate at varying frequencies within the given range. By exciting the object at each frequency within the range, and superimposing the maximum signals obtained for each pixel, the resulting image is likely to show all the locations on the object which may be defective. The preferred form of interferometry is electronic shearography, which is particularly suitable for recording interference patterns in a video buffer. The present invention is especially useful in detecting delaminations in bonded articles, and is particularly advantageous in testing large objects.

What is claimed is: 
     
       1. An apparatus for nondestructively testing an object, comprising: a) means for reflecting coherent radiation from the object, and for forming an image of the object with the reflected radiation,   b) means for perturbing the object with acoustic energy, the perturbing means comprising means for generating acoustic energy having a frequency which changes continuously from a first value to a second value, the first and second values being different, the perturbing means being operable while the coherent radiation continues to be reflected from the object, wherein a plurality of images of the object can be formed by the image-forming means while the object is being perturbed,   c) means for generating a series of composite patterns representing comparisons of images formed while the object is perturbed, with an image formed while the object is not perturbed, wherein each of said composite patterns is stored in a video buffer having a plurality of pixels, and   d) means for generating an image representing the maximum of all the composite patterns in said series, the latter generating means comprising means for forming a pattern each pixel of which is the pixel of greatest intensity of all corresponding pixels obtained from said composite patterns of said series.   
     
     
       2. The apparatus of claim 1, wherein the perturbing means comprises means for perturbing the object without any mechanical coupling to the object. 
     
     
       3. The apparatus of claim 2, wherein the perturbing means comprises means for directing acoustic energy through a gaseous medium, towards the object. 
     
     
       4. The apparatus of claim 3, wherein the gaseous medium is air. 
     
     
       5. A method of nondestructively testing an object, the method comprising the steps of: a) reflecting coherent radiation from the object, and forming an image of the object with the reflected radiation,   b) perturbing the object by directing acoustic energy towards the object, the acoustic energy having a frequency which changes continuously from a first value to a second value, the first and second values being different, the perturbing step being performed while the coherent radiation continues to be reflected from the object, so as to form a plurality of images of the object while the object is being perturbed, and   c) forming, for each image formed in step (b), a composite pattern representing the comparison of the image formed in step (a) with an image formed in step (b), wherein each composite pattern is recorded in a video buffer having a plurality of pixels, and   d) generating an image representing the maximum of all the composite patterns formed in step (c), the generating step including forming a pattern each pixel of which is the pixel of greatest intensity of all corresponding pixels obtained in the patterns of step (c).   
     
     
       6. The method of claim 5, wherein the perturbing step is performed without any mechanical coupling between the source of acoustic energy and the object. 
     
     
       7. The method of claim 6, wherein the perturbing step comprises the step of directing acoustic energy through a gaseous medium, towards the object. 
     
     
       8. The method of claim 7, wherein the gaseous medium is air.